Although it has long been recognized that the ultimate site of action of steroid hormones reside in the nuclei of target cells, only recently has it been found that steroid receptor proteins are capable of not only binding to DNA, but of distinguishing between component nucleotide bases and potentially recognizing specific DNA sequences. What this potency means in steroid regulation of gene expression is far from known. In order to pursue both the questions of receptor recognition of genomic sites and the effect such recognition has on altered transcription, we propose to isolate the mRNA for the estrogen induced rat uterine protein (IP), creatine kinase. Using our established methods of IP purification and preparation of poly- and monoclonal antibodies agains IP, we will isolate the mRNA, prepare the cDNA and identify the genomic DNA of this steroid modulated protein synthesis. There are two signal advantages of this system: 1) experimental evidence exists of tight linkage between nuclear receptor occupancy and amount of IP synthesized suggesting a direct effect of the hormone receptor on gene expression, 2) the gene product is increased in a transitory and singular model during the earliest phase of uterine development. So a careful dissection of gene structure, both coding and regulatory regions, might provide a paradigm of steroid effects involved in cellular development as opposed to those seen in terminal differentiation, i.e., oviduct egg proteins and vitellogenin. Furthermore, the immunocytochemical identification of induced creatine kinase using monospecific antibodies will aid in the cellular locus in the uterus of this estrogen modulated synthesis. Such localization may provide insight into the function of the enzyme during early uterine development.